Diaz began having discomfort in the thumb late last season and Cox says he had a splinter from a bat removed. Surgery was performed in Atlanta on Wednesday by team physician Dr. Gary Lourie.
Diaz hit .313 and set career highs with 13 homers and 58 RBIs in 2009. He entered this season with a .310 career average but was hitting only .178 in 26 games before he was placed on the 15-day disabled list Saturday.
Eric Hinske has hit .542 with two homers and 10 RBIs in seven starts in left field. He started again against Cincinnati on Wednesday night.